The proposed research is designed to improve our understanding of the evolution of developmental mechanisms in general and of morphogenetic processes in particular. The biomedical relevance of this project is two- told firstly it will help us to understand how conserved developmental mechanisms can be modified to produce the variation that is actually seen among all organisms, and secondly, this research should help us understand the developmental mechanisms underlying a variety of epithelial-mesenchymal transitions, a morphogenetic process important in the development of many tissues and organs. This project will seek to understand the basis for variation among surface mesoderm specification in anurans by characterizing the specification of surface mesoderm in three closely related frogs and one more distantly related primitive frog, by fate mapping the superficial layer of their involuting marginal zones, by doing a series of transplantation studies designed to discern the nature and origin of the specification event, and by surveying molecular markers of early tissue differentiation in an attempt to correlate expression pattern variation among the four species with the variation in the specification of their surface mesoderm, which should lead to a better understanding of the molecular signaling events involved in this morphogenetic process.